Regulatory agencies in most countries have mandated that two-way radios (also called “walkie-talkies”) be tested for Specific Absorption Rate (SAR) compliance. Two-way radios operate in simplex communication systems where the radio transmit and receive phases are interleaved in time, whereby radio transmission from the mobile station is typically triggered through the use of a Push-to-Talk (PTT) function and reception is enabled in between transmit phases. The PTT function can be typically triggered by the user pressing a PTT button located on the radio itself, which is typically done when the radio is operated in front of the face, or by pressing a PTT button residing on an audio accessory, such as a chest-worn radio-speaker microphone (RSM) wired to a belt-worn radio.
Accordingly, the International Electrotechnical Commission (IEC) 62209-2 (2010) SAR testing standard and currently applicable FCC regulations prescribe that SAR tests be conducted in two use-configurations, that is, with the radio in a talk-position “at the face” of a user, and more particularly with the radio placed 2.5 centimeters (cm) from a flat phantom representing the user head, and with the radio in a “body-worn” position placed next to a flat phantom representing the user body, wherein the radio is typically hosted in a carry accessory, for instance a belt-worn leather pouch, and operated by means of a RSM. In current standards and regulations, two exposure tiers are defined for consumer and professional radios, respectively, the latter allowing a larger SAR limit under the stipulation that professional users are aware of and trained on how to control radio frequency (RF) energy exposure (this is why this exposure tier was called “controlled” in past standards). Further, for discontinuous transmission such as PTT operation, SAR is averaged over 6 minutes under the occupational exposure tier. For simplex PTT radios, said time-averaging is implemented by allowing the assumption of 50% transmission duty-cycle (also called “talk-time”). For instance, the Federal Communications Commission (FCC) has adopted a 1.6 watt/kilogram (kg) SAR limits for the general public, applicable to consumer radios, and a 8.0 watt/kg SAR limit for occupational users, applicable to professional radios, therefore the measured SAR for a professional radio needs to be halved (50% talk-time) before comparing it with the 8.0 watt/kg limit.
Unlike cellular phones, which transmit at variable uplink transmit power depending on noise, fading, distance as demanded in real-time by the infrastructure networks they operate in, two-way radios are typically designed to be used in peer-to-peer (for example, radio-to-radio) and trunked (infrastructure-based) modes without real-time power adjustments. Therefore, to maximize communications reliability which are of the essence particularly in mission-critical (public-safety) applications, two-way radios are typically set to transmit at a set power level (typically corresponding to the maximum nominal rated power) whenever the PTT function is triggered. The IEC 62209-2 (2010) standard recommends that body-worn SAR measurements be conducted at the maximum power enabled in each manufacturer-defined intended use condition, which in the case of public safety two-way radios may include the use of carry accessories (for example, carry cases, belt clips) during SAR tests for radios that may be operated at the belt (body-worn position), and require the use of an audio accessory, such as a RSM or a PTT-capable Bluetooth radio, for the user to talk into upon triggering the PTT function.
Because carry accessories may provide smaller than 2.5 cm separation distances from the body and their metal content may support currents excited by the radio that may alter the resulting SAR level and distribution, it has been often observed that SAR levels may be higher in “body-worn” rather than in “at the face” use-configurations. Therefore, developing a two-way radio product to comply with SAR limits for the body-worn use frequently involves significant design efforts. Indeed, for given radio, antenna, audio and carry accessories designs, SAR is linearly dependent on the uplink transmit power averaged over a specified time-window, for instance 6 minutes.
Because standards are frequently revised and updated regulations promulgated, it is important to devise methods that ensure achievable SAR compliance of two-way radios even under shifting regulatory framework scenarios, especially considering that professional radio products can be available in the global marketplace for decades. Regulatory changes seldom occur in a synchronized and harmonized fashion across world regions and countries, rather they typically are implemented by national and regional regulators according to their own specific needs and timelines. Therefore, particular RF energy exposure restrictions are frequently encountered in applicable regulations in different countries or regions. Therefore, it is desirable to devise methods to ensure RF energy exposure compliance even under markedly non-uniform application of particular RF energy exposure requirements across countries and regions.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.